Since their first manufacture in the 1980s, single-mode erbium-doped fiber amplifiers (EDFAs) have steadily developed into one of the most widely used solid state laser media. EDFAs were initially used as optical amplifiers in telecommunication systems, and since then have been used as amplifiers for soliton-based communication systems, among others. These applications all have in common a lack of concern with extracting high energy and high peak power pulses from a single mode EDFA.
However, in order to consider fiber lasers as practical sources for most nonlinear optics applications, the power levels generated by convention, prior art cw or quasi-cw systems are not sufficient. For example, for the operation of a typical high-efficiency optical parametric oscillator, sub-picosecond pulses with pulse energies around 10 μJ are needed. Thus, any attempt to introduce rare earth-doped fiber lasers as amplifiers for such systems will adversely affect their operation, since the long lengths of these amplifiers will cause a wide array of prohibitive nonlinear effects and prevent the attainment of pulses with peak powers larger than 1 kW.
The use of a chirped pulse amplification (CPA) technique with fiber amplifiers is a potential solution to the above problem of low energies and powers in fiber laser sources. CPA with fiber amplifiers can successfully utilize the potential of fibers to increase pulse energies and average powers from compact fiber and laser diode sources to the levels comparable to those currently obtainable with many large-frame scientific lasers.
According to the CPA method, ultrashort pulses are stretched prior to amplification, then amplified, and finally recompressed prior to transmission. By amplifying stretched pulses of relatively long duration, the peak power in the amplifier is maintained relatively low such that nonlinear effects and pulse break-up are prevented. However, due to the physical properties of optical fibers and fiber amplifiers, there are a number of problems and limitations to be overcome for implementing CPA in fibers: nonlinear effects in fibers occurring at high peak powers, ASE-limited gain, increase in recompressed-pulse duration due to gain narrowing effect, limited output powers due to limited pump powers, recompression of pulses down to their initial duration using compact compressor and stretcher arrangements, etc. Frequently, a pulse with a large amount of chirp is generated in a fiber amplifier, and then recompressed in bulk optics. Another approach is to amplify pulses in multi-mode fiber, thereby reducing the nonlinearities, by using a mode with a large effective area.
For some applications, however, it is desirable to generate a compressed, femtosecond pulse at the end of a single mode fiber tip. For example, it is desirable to utilize short pulses in an endoscope to generate nonlinear signals for medical imaging applications. Such an application requires the smallest possible mode size at the fiber tip, so as to generate a high intensity and a large nonlinear signal. In applications making use of nonlinear fibers, such as supercontinuum generation, variations in launch conditions can be a problem when focusing into small core nonlinear fibers with bulk optics. Therefore, it is desirable to splice the nonlinear fiber directly to the amplifier output fiber, to minimize such variations. Again, a compressed pulse at the end of a single mode connector is optimal for launch into the nonlinear fiber.
One prior art arrangement for providing amplification of femtosecond pulses in single mode fiber is discussed in the article “Amplification of femtosecond pulses in a passive, all-fiber soliton source”, by D. J. Richardson et al., appearing in Optics Letters, Vol. 17, No. 22, Nov. 15, 1992, at page 1596 et seq. In this case, the fiber amplifier is characterized as exhibiting an anomalous dispersion, and as such, suffers from pulse collapse and wave breaking. Numerous prior art references further assert that the nonlinearities present in single mode fiber prevent the generation of high power pulses, and as a result either multimode fiber or bulk optic compression is required to generate short, high power pulses.